Liquid crystal displays which switch between a transparent (black) and a light scattering (white) state have insufficient back scattering efficiency to achieve adequate reflectivity in the scattering state. This is because the liquid crystal structures which scatter light do so predominantly in the forward direction, whereas efficient display operation requires light to be back scattered.
Two approaches are known in the prior art to increase the fraction of light back scattered towards the observer. One of these approaches is to increase the thickness of the liquid crystal layer. This approach however results in unacceptably thick liquid crystal layers (the thickness of which approaches the pixel dimension), which have extremely high switching voltages, yet still exhibit inadequate back scattering efficiency.
Another approach employed in the prior art to increase the fraction of light scattered back towards the observer is to utilize a prism sheet, i.e., a selectively reflective material, external to the display cell. The external prism sheet is designed to reflect none of the light transmitted by the liquid crystal layer in its transparent state. Instead, it reflects a portion of the light scattered by the liquid crystal layer in the forward direction in its scattering state back through the liquid crystal layer, where it is scattered a second time, predominantly back towards the viewer.
This approach is illustrated in FIG. 1. Specifically, FIG. 1 shows a vertical cross-sectional view through a display cell 10 as described in the prior art comprising a first transparent substrate 20, a second transparent substrate 22, a layer of liquid crystals 24 and an external prism film 26. The substrates 20 and 22 are patterned with a transparent electrode material and layers to align the liquid crystal layer. For clarity, those layers are not shown in this figure. An image 30 is created in the liquid crystal layer by the application of an electric field between transparent substrates 20 and 22. Thermal means can also be employed to generate the image. When viewed at normal incidence in the direction of arrow 32, the reflected image 34 of image 30 formed in the external prism is coincident with image 30. However, when the display is viewed at an off-normal angle, such as the direction of arrow 36, reflected image 34 is laterally displaced with respect to image 30 in the direction of arrow 38 and appears in position 40. Thus, the information presented on the display is destroyed by the reflected image from one pixel being superimposed on the primary image of an adjacent pixel or even on pixels beyond the adjacent pixels.
The latter approach, which is illustrated in FIG. 1, has the disadvantage that the image formed by reflection in the prism film is located a substantial distance behind the plane of the liquid crystal layer. This produces unacceptable parallax between the reflected and the primary images when the display is viewed from a direction which is not normal to the display plane. This is particularly problematic in the case of high density displays in which the pixel pitch is of the order of one magnitude less than the display substrate thickness.
In view of the above-mentioned drawbacks with prior art approaches for increasing the fraction of light being back scattered towards an observer, there is a continued need for developing new and improved reflective display devices which are capable of increasing the light reflected without introducing parallax between the primary and reflected images.